Describing talks with Mr Corbyn as “extensive”, the Sinn Fein leader said: “We are of one mind in terms of protecting the Good Friday Agreement. In terms of the need for a backstop.

“We have said to him that there is a backstop. It is the backstop. It is the only backstop that there is. And that it represents the bottom line for us.

“He has heard that message very, very clearly.

“We have also set out for him that in the event of a crash, in the event of a no-deal Brexit, that the Irish question will loom even larger in British politics.

“Because in those circumstances there will be a democratic imperative to reach for the Good Friday Agreement and to trigger the referendum provision in that agreement and to put the issue of the border and Irish unity to the people.”

Asked if the Labour leader agreed with Sinn Fein’s stance on a border poll, Ms McDonald said: “He understands that clearly as does Mrs May, that I’m making statements of fact here.

“It is a fact that the Good Friday Agreement provides a mechanism for a democratic decision on the border and on Irish unity.”